Protective coating and process for the prevention of halide-induced stress corrosion ot titanium alloys and other space age metals

ABSTRACT

Addition of small amounts of the nitrate ions, particularly sodium nitrate, to water solutions affords protection against halide-induced stress corrosion cracking. The nitrate may be prepared in a stable base water solution or dispersed in a suitable resin. The solution may be applied by dipping, brushing, spraying, etc.

United States Patent [21] App]. No. [22] Filed [45] Patented [73] Assignee [54] PROTECTIVE COATING AND PROCESS FOR THE PREVENTION OF HALIDE-INDUCED STRESS CORROSION OT TITANIUM ALLOYS AND OTHER SPACE AGE METALS 3 Claims, No Drawings [52] U.S. Cl 148/6.2, 21/2.7, 106/14, 148/614 [51] Int. Cl C23f 7/26, C231 9/00 [50] Field of Search 8/6 2,

[56] References Cited UNITED STATES PATENTS 2,276,353 3/1942 Thompson 148/62 2,574,225 11/1951 Ringk 117/127X 2,795,560 6/1957 Williams. 252/387 2,804,407 8/1957 Wallace 148/614 3,053,692 9/1962 Pocock..... 117/132 X 3,290,174 12/1966 Kendall 134/3 3,411,958 ll/1968 Sink 148/62 FOREIGN PATENTS 180,938 3/1966 U.S.S.R 148/614 OTHER REFERENCES Chem. Abstracts Vol. 65: 11926d Oct. 10, 1966 Handbook on Titanium Metal 7th Ed. 1953 page 40 Rem-Cru Titanium Manual 1954 Rem-Cru Sec. XVI p. 2 Titanium Inc. Copy in Op. 1 10 148- 6.2

Primary Examiner-Ralph S. Kendall AttorneyGeorge C. Sullivan ABSTRACT: Addition of small amounts of the nitrate ions, particularly sodium nitrate, to water solutions affords protection against halide-induced stress corrosion cracking. The nitrate may be prepared in a stable base water solution or dispersed in a suitable resin. The solution may be applied by dipping, brushing, spraying, etc.

PROTECTIVE COATING AND PROCESS FOR THE PREVENTION OF HALIDE-INDUCED STRESS CORROSION OT TITANIUM ALLOYS AND OTHER SPACE AGE METALS This is a continuation-in-part of application Ser. No. 638,764 filed May 16, 1967, now abandoned.

Conventionally, aircraft have been made of aluminum, which is an excellent material for ordinary temperatures, but it loses strength very rapidly above about 300 F. Thus, for highspeed supersonic aircraft operating in the temperature range of Mach 3, the strength to density ratio of aluminum alloys is reduced to an extremely low value. Present experience indicates that the titanium alloys are a much better selection for the Mach 3 transport because of their superior strength to density ratio and because they are more stable alloys than the stainless steels.

Even so, titanium does present some problems, one of which is stress corrosion induced by the presence of the halides, particularly chlorine and is evidenced by cracking, such as around spot welds, when parts or the finished article are subjected to a high temperature-high stress environment. For example, five parts per million of sodium chloride present in deionized water has been found to cause cracking of titanium in high temperature-high stress environments. However, when similar titanium specimens are treated in accordance with the invention, no cracking occurs when the specimens are exposed to water containing chlorides. The significance of the problem can be appreciated when it is realized that ordinary drinking (tap) water may contain chlorides in concentration up to about 250 parts per million. This problem is also prevalent near the seacoast and when salt is used to clear snow from runways.

Stress corrosion cracking is not only caused by salts of the halides but other compositions containing halides as well. For example, organic compounds containing chlorine such as polyvinylchloride or cutting fluids containing chlorine cause cracking.

A typical formulation which may be used as a stable base solution may be as follows:

1.0% sodium nitrate 0.1% sodium chromate 0.1% wetting agent 0.01% bactericide Distilled water to make one liter This solution is suitable for use in ordinary steel containers, tin cans, spray equipment, etc. The sodium chromate is an efficient rust inhibitor; the wetting agent, such as Tergitol TMN," a trimethyl nonyl ether of polyethyline glycol containing 6 mols of ethylene oxide, sold by Union Carbide Corporation, improves wetting or penetration between metal surfaces, and the bactericide, such as sodium dimethyldithiocarbonate, pennits long-term storage without growth of micro-organisms. This stable base solution is normally diluted with 100 parts of water, thereby resulting in about a 0.01% sodium nitrate solution. This system has been utilized in the processing of alloys prior to spot-welding and in all deionized water rinse tanks during machining operations. It is also used to treat subassemblies and finished articles. It has also been utilized for aircraft washdown for field operation.

It should be understood that above concentration is typical and in no way a limitation. ln other words, the concentration of the nitrate can be increased if found desirable. However, the suggested concentration has been found satisfactory, particularly for rinse water which must be changed frequently due to overall contamination. While distilled water is preferred to dilute the base solution, ordinary tap water has been used with good results.

A second method which provides a more permanent protective coating involves the above base stock, to which is added about an equal part of a thermally stable water-soluble or dispersable resin or about 0.5% sodium nitrate solution. One such resin which is particularly suitable is one sold by Union Carbide Corporation under the trade name Ucarsil 104," a water soluble organo functional srlrcone, a srloxane primer. A

thin film of this material will prevent stress corrosion cracking of parts exposed to NaCl solutions, even though such parts have been exposed to temperatures as high as 550 F. for in excess of hours. It has been found that this thin film will stay intact during forming operations, and typical spot-welding operation can be accomplished through the film without loss of protection. This solution may also be applied by any standard means, such as dipping, brushing or spraying.

Other wetting agents which may be used are Triton X-l 02, an octylphenoxy polyethoxyethanol, sold by Rohm and Haas Co., and a NOPCO 9092, alkylolamide plus alkylaryl sulfonate, NOPCO Chemical Company. Additional bactericides are carbolic acid (USP) and beta naphthol (USP). Other siloxane primers which have been used with excellent results are Chemlock 607" sold by Hughson Chemical Company and Y-43l0" sold by Union Carbide Corporation. Similarly, other compatible and temperature resistant resins may be used.

A particularly interesting feature of this invention is that the nitrate may be compounded with other materials containing chlorine and yet eliminate the possibility of halide induced stress corrosion. Chemlock607, for example, is chlorinated siloxane metal primer, and when mixed with the base nitrate solution, the formulation retains the effectiveness of the primer without impairing its function, and at the same time eliminates stress corrosion.

While specific embodiments of the invention have been shown and described, it should be understood that certain alterations, modifications and substitutions may be made to the instant disclosure without departing from the spirit and scope of the invention.

What is claimed is:

I. A process for prevention of halide induced stress corrosion cracking of titanium alloys comprising coating said alloys and parts made thereof with a water solution consisting essentially of nitrate ions, and wherein the concentration of nitrate ions exceeds that of the halide ions.

2. A process as defined in claim I wherein the water solution consists essentially of a mixture of l.0% sodium nitrate, 0.1% sodium chromate, 0.1% wetting agent, 0.01% bactericide, and distilled water to make one liter.

3. A process as defined in claim 1, wherein the water solution consists essentially of about 1 part of the mixture defined by claim 2, nd about 100 parts of water. 

2. A process as defined in claim 1 wherein the water solution consists essentially of a mixture of 1.0% sodium nitrate, 0.1% sodium chromate, 0.1% wetting agent, 0.01% bactericide, and distilled water to make one liter.
 3. A process as defined in claim 1, wherein the water solution consists essentially of about 1 part of the mixture defined by claim 2, and about 100 parts of water. 